In recent years, endoscopes have been widely used in both the medical and industrial fields. Recently, a capsule endoscope has been introduced for use in medical applications that can be swallowed by a patient; see for example, FIG. 1 of Japanese Laid-Open Patent Application 2001-91860 and FIG. 1 of International Patent Publication WO 01/65995 A2. Such endoscopes are advantageous in that they greatly reduce the pain associated with an endoscopic examination by avoiding the pain associated with inserting the insertion portion of a conventional endoscope.
Japanese Laid-Open Patent Application 2001-91860 discloses a conventional capsule endoscope wherein an objective lens and an illuminator, made from light emitting diodes that are symmetrically positioned about the objective lens, are provided inside a roughly hemispherical, transparent cover. A region of an object that is illuminated by the light emitting diodes is imaged by the objective lens onto an image sensor. The capsule endoscope disclosed in International Patent Publication WO 01/65995 A2 has substantially the same construction as that shown in FIG. 1 of Japanese Laid-Open Patent Application 2001-91860.
In these conventional examples, an objective lens is arranged along a central axis of a roughly-hemispherical, transparent cover that forms part of a capsule. However, when the radius of curvature of the transparent cover in a central portion of the field of view is the same as the radius of curvature near the periphery of the field of view, due to the fact that the radius of curvature of the transparent cover is determined according to the external diameter of the capsule, there is a problem in that the length of the capsule endoscope becomes excessive.
Further, in the above patent publications, nothing is mentioned at all concerning the conditions needed for providing an optimum observation field of view of a capsule endoscope.